1. Field of the Invention
This invention concerns a magnetic disk substrate, and in particular, a magnetic disk substrate with excellent heat resistance which can be made very thin, and which is suitable for the manufacture of magnetic disks of high recording density.
2. Description of the Related Art
Magnetic disk, used as a recording medium for a computer, comprises a substrate and a magnetic film formed on the substrate. The substrate is required to have the following properties:
(a) Its surface properties after precision machining, precision grinding or precision polishing must be satisfactory so that magnetic heads can run smoothly over the disk, and such that stable magnetic properties are obtained with few magnetic errors.
(b) There must be no protuberants or holes in its surface which could lead to defects in the magnetic film formed on the substrate.
(c) It should have sufficient strength and rigidity to withstand high speed rotation when the disk is used, and precision machining during manufacture.
(d) It must be able to withstand the heat applied when forming the magnetic film.
(e) It must be lightweight and non-magnetic.
(f) It must have some degree of corrosion resistance.
Conventionally, Al-Mg or other aluminum alloys are used as substrate materials satisfying these conditions.
Recently, there has been a trend in magnetic disks toward higher recording densities and greater compactness, so the following conditions must also be met:
(A) The magnetic film must have improved magnetic properties, for example it must be possible to form a film with high coercive force.
(B) The magnetic film must be made thinner.
(C) The flying hight of the magnetic heads must be lessened.
(D) The gap length of the magnetic heads must be reduced.
(E) Techniques for positioning the magnetic heads must be improved.
(F) The substrate must be made more compact and thinner.
In order to satisfy these requirements, various provisions are conventionally adopted.
A magnetic film of high recording density is formed by sputtering (Kyuzo Nakamura, "Kinzoku" (Metals), Nov. 1986), or the substrate is made resistant to the heat applied during sputtering.
To lessen the flying hight of the magnetic heads, the aluminum alloy disk is given a Ni-P plating to cover inclusions. In this way, high level surface properties are obtained with little surface roughness and undulations (Masahiro Saito, "Jitsumu Hyomen Gijutsu" (Practical Surface Technology), Vol. 35, No. 6, 1988).
Further, alloy compositions with few inclusions that could lead to electrical and magnetic errors are used for the substrate (Unexamined Published Japanese Patent No. 63-216953, and attempts are made to reduce the thickness of the substrate itself.
To satisfy the above conditions (a)-(f), new materials are constantly being developed to replace aluminum alloys. Examples of these are glass (Hiroyoshi Ishizaki, "Kougyo Zairyo" (Engineering Materials), Vol. 35, No. 5), ceramics (Takeshi Matsumoto, "Jisei Zairyo Kenkyukai" (Magnetic Material Research Group), "Denshi Zairyo Hyomen Shori Gijutsu" (Electronic Material Surface Treatment Technology), July 1987, and titanium (Published Unexamined Japanese Patent Applications Nos. 52-105804, 59-151335 and 63-142521).
Al-Mg or other alloys conventionally used as magnetic disk substrates however have the disadvantage that they do not have sufficient heat resistance to withstand the substrate temperature rise produced by sputtering, which is carried out to form a magnetic film with a high coercive force and low thinness required for high recording density. Further, although the composition of this material was adjusted, it still contained essentially a large quantity of inclusions and high level surface properties could not be obtained. It is therefore impossible to achieve the reduction of the head flying hight required for high recording density, and achieve a sufficient reduction of electrical errors.
To obtain high level surface properties, the surface is plated with Ni-P as described above, but the method has the disadvantage that the plating yield is fairly low. Further, the Ni-P plating tends to crystallize and retain its magnetism at the fairly low temperature of about 350.degree. C., and peel off.
Further, if the aluminum alloy substrate is made thin according to demand, sufficient strength and rigidity cannot be guaranteed to enable it to withstand the high speed disk rotation (for example 3600 rpm) required for a short access time.
Further, if glass or ceramics were used as substrate materials, sufficient heat resistance and strength are obtained, but both materials are brittle and are therefore not very reliable. In the case of glass, moreover, gas discharges occurred due to the high temperature applied for sputtering, and elemental impurities in the glass diffused into the magnetic film, whereby the magnetic properties of the film are adversely affected. Ceramic substrates, on the other hand, contain a large number of pores, and sufficient surface properties cannot be obtained. Glass and ceramics are therefore still unsatisfactory as substrate materials.
As mentioned above, various proposals have been made regarding titanium substrates. In said Published Patent Application No. 52-105804, for example, a magnetic disk substrate is proposed based on titanium or titanium alloy with an oxide or nitride film formed on the surface. This film is formed by oxidizing or nitriding the surface, thereby increasing surface hardness and improving machinability so as to obtain satisfactory surface properties. Further, Published Unexamined Japanese Patent Application No. 59-151335 proposes a magnetic disk substrate of a type titanium alloy containing no less than 80% of hcp phase in terms of volume fraction, and of strength no less than 60 kg/mm.sup.2.
Further, ordinary pure titanium and a type titanium alloys unavoidably contain .beta. stabilizing elements such as Fe and V to the extent of 0.1 weight %. These .beta. stabilizing elements easily segregate in the melting, blooming and rolling steps, and easily give rise to pitting due to segregation during polishing. If it is attempted to obtain high level surface properties, therefore, the yield declines. Further, the rate of oxidation and nitriding is different in the .alpha. and .beta. phases (segregation band), and it is difficult to make oxidizing or nitriding uniformly. The yield of substrates coated with an oxide or nitride film is therefore lower than that of substrates consisting only of titanium, and manufacturing costs are higher. It is stipulated that the total content of elemental impurities in ordinary titanium or a type titanium should not exceed 0.3 weight %. This level of impurities is much lower than that in aluminum alloys, however it is still not sufficiently low to eliminate all the errors due to inclusions, and to obtain high level surface properties.
Further, the magnetic disk substrate disclosed in Published Unexamined Japanese Patent Application No. 63-142521 comprises a core layer of titanium or titanium alloy, an insert layer, a layer of nickel, titanium, nickel alloy or titanium alloy, and a layer of glass or ceramics superimposed on one another in that order. As the number of processes required to manufacture such a substrate is greater than in the case of titanium alone, however, manufacturing costs are high and manufacturing time is also lengthy. Further, if ceramics is used for the outermost layer, adequate surface properties cannot be obtained since ceramics contain a large number of pores. If glass is used, the surface roughness Ra (explained later) can be kept to the very low level of approx. 0.005 .mu.m, but this solution is undesirable as the heads tend to stick to the glass surface.
At present, therefore, a satisfactory magnetic disk substrate cannot be obtained even if titanium is used as the principal material.